All Posts are placed into one or several Categories. A main category is always required. Additional categories are called "Extra categories".
Categories can be organized into hierarchies of Sub Categories with an infinite nesting level.
Since it is impossible to place a Post into a Collection without going through a Category, all new collections are created with at least one default category, which may be called "Uncategorized". Without this category it would be impossible to place a Post into the collection.
Even if you configure b2evolution not to display radio buttons for the main category, a main category will always be assigned internally.
The main category is also always considered as an extra category internally. Therefore, selecting a main category through a radio button will also check the extra-category checkbox. Doing this allows for faster database requests when finding the posts that belong to a specific category or even a specific collection.
Created by fplanque • Last edit by fplanque on Aug 20, 2016
It’s just a detail, but could you please explain in the manual why the extra category is always selected along with the main category? When something is in a certain main cat, it doesn’t require the same extra cat, does it?